The invention relates to a developing apparatus for turning an electrostatic latent image on an image retainer into a visible toner image, more particularly to a developing apparatus which reduces the generation of deposition or fusion-bonding of toner particles on bearing sections of rotary members such as a developing roller and a stirring and conveying screw, and a process cartridge which is equipped with the developing apparatus.
An electrophotographic image-forming apparatus forms an image by charging the surface of an image retainer uniformly by a charger and running an exposure unit according to a document or image data to form a latent image on an image retainer. Then, the image forming apparatus feeds a single-component developer which contains a toner only or a two-component developer which contains both toner and carrier to a developing zone by turning the latent image into a toner image on the image retainer by a contact or non-contact developing manner, causes a transfer unit to transfer the toner image onto a transfer material such as a paper sheet, and thermally fix the toner image on the transfer material on the transfer material.
The developing apparatus for developing a latent image into a toner on the image retainer is equipped, for example, with a developing sleeve made of a cylindrical member which can rotate, stationary magnets provided in the developing sleeve, and a stirring member which stirs the two-component developer and applies electric charges adequate for development. The developing apparatus causes the developing sleeve to retain the charged two-component developer which is attracted by the magnet on the circumferential surface of the sleeve and feed the developer from the surface of the developing sleeve sequentially to the developing zone while the developing sleeve rotates to make the latent image visible on the image retainer. After development, the developer left on the developing sleeve is automatically removed from the developing sleeve by repulsive actions of magnetic fields generated by arrangement of magnet poles of the built-in magnets.
Various rotary members such as the developing sleeve in the developing apparatus are rotated to carry a developer and supply the developer to the developing sleeve. These rotary members are rotatably supported by bearing sections in the developing apparatus.
FIG. 13 shows a partial sectional view of a sample structure of a bearing section in a conventional developing apparatus.
Referring to FIG. 13, numeral 46 is a developer container which contains a developer and holds various rotary members such as a developing sleeve. Numeral 461 is a bearing section fixed to the side wall of developer container 46. Numeral 432 is an axis of a rotary member whose end is fitted with shaft section 433 in a body. The shaft section 433 has a large-diameter part F and a small-diameter part E. The small-diameter part E is rotatably fit into hole H of bearing section 461 which is fixed to the side wall. The axial-movement of the rotary member-is limited by the further end face of bearing section 461 (which is away from the side wall of developer container 46) and the end face of the large-diameter part F opposite to the end face of bearing section 461. In other words, the axial movement of the rotary member is limited by the sliding surface which is the whole end face of bearing section 461.
By the way, when the developer enters the bearing section which holds the rotary member in the developing apparatus, the developer may be broken by sliding inside the bearing section. In the example of FIG. 13, the sliding surface is the whole end face of bearing section 461 and may break the developer easily.
To solve this problem, various measures have been taken to prevent the developer from entering the sliding surface. Substantially, conventional technologies have employed sealing members or filling materials to shut out developer from the clearance of the bearing section (i.e. see Patent Document 1), structures which do not allow developer to go into the bearing section together with a sealing member (i.e. see Patent Document 2), and technology to prevent invasion of the developer into the bearing section by using distribution of magnetic flux density of the developing sleeve (i.e. see Patent Document 3). As just described above, the conventional technologies have improved developing apparatus, based on the concept to prevent developer from entering the bearing sections.
However, it is very difficult to completely prevent invasion of fine toner particles in microns. As the period of image formation service becomes longer, toner particles have deposited in the bearing sections little by little. Further, recently, as the digital electro-photographic technology has advanced, high-quality toner images have been demanded. On this demand, technologies to produce chemical toners as typified by polymerization toners have become conspicuous. Thanks to these technologies, we can produce small toner particles of some microns in diameter. Further, on environmental concerns, image forming apparatus are requested to save energy and at the same time image forming technologies to fix images at low temperature have been demanded. One of such technologies has provided toners whose softening point is much lower than the softening points of conventional toners. (For example, see Patent Document 4.)
However, no technology has been established to completely prevent toner particles from entering bearing sections of rotary members in developing apparatus. Furthermore, the above fine toner particles of some microns in diameter have made the invasion problem more actualized. Particularly, small-diameter toners for low-temperature fixing can easily invade the bearing sections and are easily coagulate and fusion-bonded by little frictional heat.
Recent image forming apparatus have been demanded to be smaller and faster. Therefore, parts are densely arranged near the developing unit. This reduces the efficiency of heat radiation and makes the invasion and fusion-bonding problems more serious. Further, to meet the increasing demand for color printouts at offices, various kinds of color image forming apparatus have been developed vigorously. However, the color image forming apparatus must be equipped with plural developing units, which makes the components arranged densely around the developing units. Consequently, it has been earnestly demanded to solve the problems of toner coagulation and fusion-bonding in the developing units.
[Patent Document 1] Japanese Non-Examined Patent Publication H10-198163
[Patent Document 2] Japanese Non-Examined Patent Publication 2000-88108
[Patent Document 3] Japanese Non-Examined Patent Publication H05-297721
[Patent Document 4] Japanese Non-Examined Patent Publication 2000-214629
The above problems are caused by invasion of toner particles into the trapped sliding space between the rotary member shaft and the bearing section and grinding of toner particles by the sliding surfaces. The present inventors inferred that coagulation and fusion-bonding of toner particles are accelerated because the sliding space in the bearing section is densely filled up with toner particles and that the inventors can possibly solve the problem by reducing the sliding surfaces which grind the invading toner particles even when the toner particles are for low-temperature fixing. After a long trial-and-error process, we have reached the invention. In other words, unlike the concept of the conventional technologies which prevents invasion of toner particles into bearing sections, the invention uses a concept of suppressing coagulation and fusion-bonding of toner particles by reducing the sliding surfaces between the rotary member shaft and the bearing section since fine toner particles invade the sliding surfaces through the clearance of the bearing section and are ground there.